The problems of defining the pathways of transepithelial water flow will be approached using mass spectrometry adapted to provide spatial resolution of water flux from tissue. High sensitivity provided by this technique should permit sampling of the concentration profile of water tracer in the "unstirred" layer adjacent to an epithelial membrane, allowing the evaluation of the relative magnitude of transjunctional and transcellular water flux at the lumenal surface. "High impedance" probes will be prepared from micropipettes of 2 micrometers outer diameter, plugged with dimethyl silicone to permit diffusive flow of water into the vacuum chamber of a mass spectrometer without tissue damage. The application of H2O18 to the "serosal" surface of a membrane will establish a concentration profile of tracer water at the "lumenal" surface, conforming to the pathways of transmembrane water flow. Initial studies employing Nucleopore, synthetic membranes with well characterized pore size, will define the degree of spatial resolution available and optimal operating conditions. Subsequent experiments will be carried out in Necturus gallbladder, a well studied loose epithelium with large cells and other desirable characteristics facilitating its study. Studies will be done in the presence and absence of isotonic fluid transport, during inward and outward osmotic flow, and in the presence of various agents influencing water transport.